As cars are built today to last ten or fifteen years and be driven 100,000 to 200,000 miles, brakes usually need to be resurfaced at least once during the life of every car. Most owners do not become aware of the need for brake repair until the brakes make a sound indicating wear beyond the useful depth of braking surface. By this time, the brake drum, against which the movable shoe engages for braking action, becomes scored. Then, the drum must be machined to present a round, smooth surface for the new brake shoe surface to engage.
In order to machine the brake drum to present a round, smooth surface, calipers have been developed to accurately measure the inside diameter of the brake drum. It is essential to have a constant inside diameter in the refinished drum to engage the shoe surface evenly. U.S. Pat. No. 5,317,814 shows a drum caliper that can accurately measure diameter only if the points 44 and 74 of the caliper are at the same depth within the drum. If the beam is not precisely parallel to the drum, the measurement of diameter will be imprecise.
U.S. Pat. No. 5,465,501 was intended to correct the problem of having a skewed plane 38 (FIG. 3) giving an inaccurate reading. However, the slidable pins 26 and spring detents or other locking mechanisms described in the patent become fouled in an environment where metal shavings are ubiquitous. If a pin inadvertently slides inwardly within the drum, instead of resting on the periphery of the drum, a skewed angle can result with imprecise measurements. There is a need for fixed stops for precise depth locations, rather than slidable stops that can slip off the periphery of the drum.